TreeMap in Java

A TreeMap in Java is a part of the java.util package that implements the Map interface. It stores key-value pairs in a sorted order using either a natural or custom comparator.

• TreeMap internally uses a Red-Black Tree for efficient sorting.
• Provides O(log n) time for insertion, deletion and lookup due to its self-balancing Red-Black Tree structure.
• TreeMap does not allow null keys, but allows null values.

TreeMap elements: {}

Hierarchy of TreeMap

TreeMap extends AbstractMap and implements the NavigableMap, SortedMap, Map, Cloneable, and Serializable interfaces.

Constructors of TreeMap

In order to create a TreeMap, we need to create an object of the TreeMap class. The TreeMap class consists of various constructors that allow the possible creation of the TreeMap. The following are the constructors available in this class:

1. TreeMap()

This constructor is used to build an empty TreeMap that will be sorted by using the natural order of its keys. 

Syntax:

TreeMap<K, V> map = new TreeMap<>();

TreeMap using TreeMap() constructor
TreeMap: {10=Geeks, 15=For, 20=Geeks}

2. TreeMap(Comparator comp):

This constructor is used to build an empty TreeMap object in which the elements will need an external specification of the sorting order.

Syntax:

TreeMap<K, V> map = new TreeMap<>(Comparator<? super K> comparator);

TreeMap sorted by roll number: {111 Geek1 New York=1, 121 Geek3 Paris=3, 131 Geek2 London=2}

3. TreeMap(Map M)

This constructor is used to initialize a TreeMap with the entries from the given map M which will be sorted by using the natural order of the keys.

Syntax:

TreeMap<K, V> map = new TreeMap<>(Map<? extends K, ? extends V> m);

TreeMap using TreeMap(Map) Constructor
TreeMap: {10=Geeks, 20=For, 30=Geeks}

Performing Various Operations on TreeMap

1. Adding Elements

We can use the put() method to insert elements to a TreeMap. However, the insertion order is not retained in the TreeMap. Internally, for every element, the keys are compared and sorted in ascending order. 

TreeMap with raw type: {1=Geeks, 2=For, 3=Geeks}
TreeMap with generics: {1=Java, 2=Programming, 3=Language}

2. Changing Elements

To change the element in a TreeMap, simply use the put() method again with the same key and the new value.

{1=Geeks, 2=Geeks, 3=Geeks}
{1=Geeks, 2=For, 3=Geeks}

3. Removing Element

We can use the remove() method to remove element from the TreeMap.

{1=Python, 2=C++, 3=Java, 4=JS}
{1=Python, 2=C++, 3=Java}

4. Iterating Elements

There are multiple ways to iterate through the Map. The most famous way is to use a for-each loop and get the keys. The value of the key is found by using the getValue() method.

1 : Geeks
2 : For
3 : Geeks

Methods of TreeMap